Note:If everyone can vote, registered or unregistered, then we might need to track some other user attributes, for instance:

Date/time stamp

IP

User agent

User cookie attributes

Besides Referendum_id, question_id, answer_id, vote_id, etc

But even this is easily “tamperable” if user changes any of the above fields, or use a Tor-like software. In that case we would need to look into some pattern recognition algorithms or research some other techniques to track user.